Optical fiber amplifiers capable of directly amplifying weak optical signals without conversion of optical signals into electrical signals are known. Such optical fiber amplifiers are typically designed by using an optical fiber doped with a special material such as erbium, and the amplification of weak optical signals is achieved by pumping the erbium-doped optical fiber with a laser. Since this type of erbium-doped optical fiber amplifiers can achieve high gain amplification over a wide range of wavelengths, they have been widely adopted in optical preamplifiers, optical power amplifiers, optical repeaters, and other essential components of optical communication systems based on the 1.55 .mu.m wavelength. Such erbium-doped optical fiber amplifiers are especially useful in wavelength division multiplex (WDM) communication systems due to their capability to simultaneously amplify optical signals of different wavelengths with little crosstalk.
However, in WDM optical communication systems, the light of a channel signal corresponding to a particular wavelength is arbitrarily modulated in intensity irrespective of other channels during its input or output from an optical fiber communication cable. Consequently, the intensity of the overall signal light input to the optical fiber amplifier varies over a fairly wide range during its operation. In this case, the optical fiber amplifier undergoes gain variations depending on the intensity of the overall input signal light due to gain saturation characteristics, that result in fluctuations of the output light. Such gain instability causes disturbances in the operation of communication systems and is an art recognized problem.
In order to alleviate this problem, a conventional method has been used, which causes the optical fiber amplifier to oscillate at wavelengths other than that of the signal light which produces a laser operation at the wavelength of oscillation. Based on the well known principle that the intensity variation of the oscillation light depends on the intensity variation of the input signal light, the optical fiber amplifier can be stabilized to maintain a constant gain, irrespective of the intensity variation of the input signal light. For example, a gain control method based on a ring resonator type laser, wherein a portion of the output light is fed back to the input port so that loss can be adjusted in the ring is described in an article entitled "Gain control in erbium-doped fibre amplifiers by an all-optical feedback loop" by M. Zirngibl, Electron Lett., 1991, 27, (7), pp.560-561, and in an article entitled "Automatic optical-loss compensation with Er-doped fibre amplifier" by H. Okamura, Electron Lett., 1991, 27, (23), pp.2155-2156.
An alternative method for controlling the gain of the optical fiber amplifier is disclosed in an article entitled "Optical gain control of erbium-doped optical fibre amplifier using an optical fiber grating" by H. K. Park, Korean Optics., February 1997, 8, (1), pp.58-62. In this method, a ring resonator, based on an optical fiber grating and an optical coupler, is used to control gain by tuning the coupling ratio of the optical coupler. Also, a method of using a Fabry-Perot type resonator, which incorporates optical fiber gratings between the input and output ports of the optical fiber amplifier and which oscillates at the wavelengths of reflection of the two optical fiber gratings, is disclosed in an article entitled "Gain control in erbium-doped fibre amplifiers by lasing at 1480 nm with photoinduced Bragg gratings written on fibre ends" by E. Delevaque et al., Electron Lett., 1993, 29, (12), pp.1112-1114.
In the ring type oscillator, however, an optical coupler for use in the ring type resonator, a variable optical attenuator for tuning loss in the resonator and a band-pass filter for selecting the wavelength of oscillation are required. Consequently, the problems arise which are associated with complexity resulting from the increased number of required components and with light loss. Although it is possible to simplify the structure of the optical fiber amplifier according to the Delevaque's method using two optical fiber gratings, gain cannot be easily controlled due to the failure to tune resonator loss. There is a need in the art for an apparatus for controlling gain in optical fiber amplifiers which sloves the problems identified above.